linux-kernel - Re: x86: A fast way to check capabilities of the current cpu

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Message-ID: <20110121172836.GK2832@htj.dyndns.org>
Date:	Fri, 21 Jan 2011 18:28:36 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Pekka Enbeerg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	akpm@...ux-foundation.org
Subject: Re: x86: A fast way to check capabilities of the current cpu

On Fri, Jan 21, 2011 at 11:21:02AM -0600, Christoph Lameter wrote:
> > I don't think percpu_read_stable() can be used here.  It's not
> > guaranteed to be stable across different cpus.
> 
> Why would that matter? The caller has to disabled preemption anyways since
> otherwise the processor may change which means that the result of the
> operation is useless.

Because

	preempt_disable();
	this_cpu_has();
	preempt_enable();
	preempt_disable();
	this_cpu_has();
	preempt_enable();

might malfunction.  percpu_read_stable() is pretty much applicable
only to stuff local to the thread.

> > Also, can we just implement what's necessary on top of this_cpu_has()?
> > this_cpu_has() already has constant handling, so there's no need to
> > add this_cpu_test_bit() at this point.
> 
> Not sure what you mean. this_cpu_test_bit is necessary because
> test_cpu_cap expects a regular pointer and performs a regular load.
> this_cpu_constant_test_bit handles the segment prefix necessary for a per
> cpu load.
> 
> The constant refers to the bit.

Oh, you're right.  Sorry about that.  Can you please then add a
comment noting that the operation is x86 only?  Maybe prefix it with
x86_?

Thanks.

-- 
tejun
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